The majority of buildings are constructed with cementitious materials or systems that vary widely in composition, design, and end use. As used herein, the term "cementitious system" refers to materials which, when mixed with an aqueous medium, bind, or impart an adhesive or cohesive behavior. Some examples include portland cements that are produced by burning limestone and clay, natural and artificial pozzolanic cements (pozzolanic materials react with lime to form calcium silicate hydrates), slag cements, combinations of portland cement and granulated blast furnace slag, refractory cements (e.g., rapid set cements containing primarily calcium aluminate compounds, such as, for example, Ciment Fondu produced by Lafarge, and Luminite, produced by Lehigh Cement Company), gypsum, expanding cements, fly ash, as well as so called "mud to cement" systems, whereby a drilling mud is converted into a cementitious material during the completion process of certain subterranean wellbores, and the like. See, for example, Bye, G. C., "Portland Cement, Composition, Production and Properties", Pergamon Press, New York, London, Ontario, Paris, Oxford (1983); Smith, Dwight K., "Cementing", Monograph Volume 4, Published by The Society of Petroleum Engineers, New York and Richardson, Tex. (1987).
Fresh cement paste is composed of cementitious material and water, while fresh concrete paste is comprised of a wide range of materials such as portland cement, fly ash, silica fume, sand, aggregate, i.e., small rocks and the like. Mixing, transporting, and placing the fresh concrete presents a number of challenges as the paste must remain highly fluid and ideally should provide homogenous transport of all particles. This problem is compounded because excessive water is frequently added to the mixture in efforts to enhance flow. The hydration of portland cement, for example, typically requires some 25-28 percent water basis weight of cement (BWOC). Unfortunately, excessive water can lead to a number of problems, such as, for example, bleed, sedimentation, reduced strength and durability, and poor bonding to structural reinforcement members.
Two types of materials used in admixture with cementitious formulations to enhance fresh paste flow, without the need to employ additional water, are water reducers and superplasticizers. However, admixtures containing either material can induce excessive bleed and sedimentation. As used herein "bleed" refers to free water collection on the surface, while "sedimentation" refers to the segregation of particle size whether during placement, or static. Excessive bleed can cause many problems, such as, for example, by reducing the durability and strength of the desired bond. In some cases, bleed water channels form on horizontal structural components, thereby reducing bond strength and creating corrosion sites. Aggregate segregation reduces the surface wear properties causing increased maintenance costs.
Recent technologies have provided a new class of cement additives, the so called rheological modifiers, or viscosity modifying agents (VMA). This class of additives is usually water-soluble polymers and function by increasing the apparent viscosity of the mix water. This enhanced viscosity facilitates uniform flow of the particles and reduces bleed, or free water formation, on the fresh paste surface. Underwater concrete placement designs frequently require a polymer admixture to reduce fines loss during placement (Khayat, Kamal Henri, "Effects of Antiwashout Admixtures on Fresh Concrete Properties", Published in the ACI Structural Journal, Title No. 92-M18, March-April, (1995)). Unfortunately, this also increases the resistance of the fresh cement paste to flow and may induce excessive frictional pressure during conveyance.
Accordingly, there is still a need in the art for methods to treat the above-described problems of bleed, sedimentation, flow resistance, etc, encountered with existing cementitious formulations.